AN1262 Methodology Datasheet

AN1262 Datasheet, PDF, Equivalent


Part Number

AN1262

Description

Offline Flyback Converters Design Methodology

Manufacture

ST Microelectronics

Total Page 30 Pages
Datasheet
Download AN1262 Datasheet


AN1262
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AN1262
APPLICATION NOTE
OFFLINE FLYBACK CONVERTERS DESIGN
METHODOLOGY WITH THE L6590 FAMILY
by Claudio Adragna
The design of flyback converters is quite a demanding task that requires SMPS engineers to cope with sev-
eral problem areas such as magnetics, control loop analysis, power devices, as well as regulations concern-
ing safety, EMC and the emerging standby consumption requirements. Lots of variable are involved and
complex tradeoffs are necessary to meet the goal.
In this scenario, the high-voltage monolithic switchers of the L6590 family greatly simplify the task and, at
the same time, allow to build robust and cost-effective low-power systems.
In this application note, after a review of flyback topology, a step-by-step design procedure of an offline sin-
gle-output flyback converter will be outlined. As an example, the design of the test board will be carried out
in details.
1 FLYBACK BASICS
Flyback operation will be illustrated with reference to the basic circuit and the waveforms of fig. 1. It is a two-
step process. During the ON-time of the switch, energy is taken from the input and stored in the primary winding
of the flyback transformer (actually, two coupled inductors). At the secondary side, the catch diode is reverse-
biased, thus the load is being supplied by the energy stored in the output bulk capacitor.
Figure 1. Flyback Topology and associated waveforms.
Vin
Is
Lp
Vout
Ls
Vac
n:1
Vcc DRAIN
L6590
L6590D
L6590A
2.5 V
OSCILLATOR
Clock
Max. Duty cycle
S
RQ
Driver
1
+
VFB E/A
-
+
PWM
-
Clock
+
OCP
-
1/100
Rsense
LEB 0.5 V
Ip
FREQUENCY
COMPENSATION
COMP
GND
ISOLATED
FEEDBACK
CLOCK
CLOCK
CLOCK
May 2001
Q
Ip
Is
Vdrain
Vin
DCM operation
Q
Ip
Is
nVout
Vdrain
TRANSITION
Q
Ip
Is
Vdrain
Ip
CCM operation
1/42

AN1262
AN1262 APPLICATION NOTE
When the switch turns off, the primary circuit is open and the energy stored in the primary is transferred to the
secondary by magnetic coupling. The catch diode is forward-biased, and the stored energy is delivered to the
output capacitor and the load. The output voltage Vout is reflected back to the primary through the turns ratio n
(VR, reflected voltage) and adds up to the input voltage Vin, giving origin to a much higher voltage on the drain
of the MOSFET.
Flyback is operated in DCM (Discontinuous Conduction Mode) when the input -or primary - current starts from
zero at the beginning of each switching cycle. This happens because the secondary of the transformer has dis-
charged all the energy stored in the previous period. If this energy transfer is not complete, the primary current
will start from a value greater than zero at the beginning of each cycle. Then flyback is said to be operated in
CCM (Continuous Conduction Mode). DCM is characterized by currents shaped in a triangular fashion, whereas
CCM features trapezoidal currents.
The boundary between these two types of operation depends on several parameters. For a given converter,
that is, as the switching frequency, inductance of the primary winding, transformer turns ratio and regulated out-
put voltage are defined, it depends on the input voltage and the output load.
At design time, whether the converter will be operated in CCM or in DCM and where the boundary will be located is
up to the designer. Usually CCM is selected with the objective of maximizing converter's power capability or minimiz-
ing primary RMS current. However, in CCM operation the system's dynamic behavior is considerably worse.
Usually, the converters based on the L6590 family devices are able to deliver the desired output power even
with DCM operation, thus CCM will not be considered.
Table 1. Converter specification data and pre-design choices
Converter Electrical Specification
VACmin
Minimum mains voltage
VACmax
Maximum mains voltage
fL Mains frequency (@ min. mains)
NH Number of holdup cycles
Vout
Vout%
Regulated output voltage
Percent output voltage tolerance )
Vr% Percent output voltage ripple
Poutmax
Maximum output power
η Expected converter efficiency
Tamb
Maximum ambient temperature
Pre-design Choices
VR Reflected voltage
ηT
Vspike
Transformer efficiency
Leakage inductance overvoltage
Vcc IC supply voltage
VF Secondary diode forward drop
VBF Bridge Rectifier + EMI filter voltage drop
2/42


Features www.DataSheet4U.com AN1262 APPLICATION NOTE OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY by Cl audio Adragna The design of flyback co nverters is quite a demanding task that requires SMPS engineers to cope with s everal problem areas such as magnetics, control loop analysis, power devices, as well as regulations concerning safet y, EMC and the emerging standby consump tion requirements. Lots of variable are involved and complex tradeoffs are nec essary to meet the goal. In this scenar io, the high-voltage monolithic switche rs of the L6590 family greatly simplify the task and, at the same time, allow to build robust and cost-effective low- power systems. In this application note , after a review of flyback topology, a step-by-step design procedure of an of fline single-output flyback converter w ill be outlined. As an example, the des ign of the test board will be carried o ut in details. 1 FLYBACK BASICS Flybac k operation will be illustrated with reference to the basic cir.
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